1. Field
The present disclosure relates to redox flow batteries, and more particularly, to redox flow batteries with high energy density and high charge and discharge efficiency.
2. Description of the Related Art
Typical secondary batteries convert electric energy into chemical energy during charging, and convert chemical energy into electric energy during discharging.
Likewise, redox flow batteries (“RFB”) convert chemical energy into electrical energy or vice versa during charging and discharging. However, unlike typical secondary batteries, an electrode material of an RFB, which stores energy, exists in a liquid state, not a solid state. Thus, in a redox flow battery, the electrode active material is stored in a tank, and a voltage of the battery may be increased by stacking unit cells.
In detail, a catholyte and an anolyte function as an electrode active material, and typically, a transition metal oxide dissolved in a solvent, so the electrode active material exists in a liquid state. That is, a catholyte and an anolyte are stored in a tank as a mixture of reduced and oxidized electrolytes.
Also, like a fuel cell, a cell generating electric energy, has a structure of carbon electrode/membrane/carbon electrode. In such a cell, the catholyte and the anolyte supplied by a pump each undergo an oxidation or reduction reaction on a surface of a corresponding carbon electrode, generating an electromotive force corresponding to Gibbs free energy. The carbon electrodes do not directly participate in a reaction and only aid the oxidation and reduction of an active material. The membrane does not directly participate in a reaction, however, it quickly delivers ions, thus playing a role of a charge carrier between the catholyte and the anolyte, prevents a direct contact between a cathode and an anode, and suppresses the crossover of active ions dissolved in the catholyte and the anolyte.
In the case of redox flow batteries, including an organometallic coordination compound as an electrolyte, a redox couple with high voltage or a redox couple with high solubility may be used to achieve high energy density.
However, a conventional redox flow battery using an aqueous solvent has a disadvantage of low energy density caused by a low driving voltage as an operation potential is limited to the water decomposition potential. Accordingly, there is a continuous demand for a redox flow battery having a high energy density and excellent charge and discharge efficiency.